The present invention relates to a surface which reflects radio-frequency, including microwave radiation, and which imparts a phase shift to the reflected wave which is electrically tunable, using liquid crystals or other electrically tunable medium.
There is an existing need for materials and/or surfaces which can steer (or focus) a radio frequency electromagnetic beam. Such materials and/or surfaces can be very useful in various applications such as radio frequency communication systems, including satellite communication system.
The present application is related to (i) U.S. patent application Ser. No. 09/537,923 entitled xe2x80x9cA Tunable Impedance Surfacexe2x80x9d filed Mar. 29, 2000 (ii) U.S. patent application Ser. No. 09/537,921 entitled xe2x80x9cAn End-Fire Antenna or Array on Surface with Tunable Impedancexe2x80x9d filed Mar. 29, 2000 and to (iii) U.S. patent application Ser. No. 09/520,503 entitled xe2x80x9cA Polarization Converting Radio Frequency Reflecting Surfacexe2x80x9d filed Mar. 8, 2000 the disclosures of which are all hereby incorporated herein by this reference. U.S. patent application Ser. No. 09/537,923 for a xe2x80x9cTunable Impedance Surfacexe2x80x9d describes a method and apparatus for mechanically tuning the surface impedance of a Hi-Z surface and thus its reflection phase using various mechanical methods. By programming the reflection phase as a function of position on this surface, the reflected beam can be steered or focused
Prior art approaches for radio frequency beam steering generally involve using phase shifters or mechanical gimbals. With the present invention, beam steering is accomplished electronically using variable capacitors, thus eliminating expensive phase shifters and unreliable mechanical gimbals. Furthermore, the reflective scanning approach disclosed herein eliminates the need for a conventional phased array, with separate phase shifters on each radiating element. The tunable surface disclosed herein surface can serve as a reflector for any static, highly directed feed antenna, thus removing much of the complexity and cost of conventional, steerable antenna systems.
It is known in the prior art that an ordinary metal surface reflects electromagnetic radiation with a xcfx80 phase shift. However, a Hi-Z surface of the type disclosed in U.S. provisional patent application Ser. No. 60/079,953 is capable of reflecting radio frequency radiation with a zero phase shift.
A Hi-Z surface, shown in FIG. 1, consists of an array of metal protrusions or elements 12 disposed above a flat metal sheet or ground plane 14. It can be fabricated using printed circuit board technology, in which case the vertical connections are formed by metal vias 16, which connect the metal elements 12 formed on a top surface of a printed circuit board 18 (see FIG. 2) to a conducting ground plane 14 on the bottom surface of the printed circuit board 18. The metal elements 12 are arranged in a two-dimensional lattice, and can be visualized as mushrooms or thumbtacks protruding from the flat metal ground plane surface 14. The maximum dimension of the metal elements 12 on the flat upper surface is much less than one wavelength (xcex) of the frequency of interest. Similarly, the thickness of the structure measures also much less than one wavelength of the frequency of interest.
The properties of the Hi-Z surface can be explained using an effective media model, in which it is assigned a surface impedance equal to that of a parallel resonant LC circuit. The use of lumped parameters to describe this electromagnetic structure is valid when the wavelength of interest is much longer than the size of the individual features, such as is the case here. When an electromagnetic wave interacts with the Hi-Z surface, it causes charges to build up on the ends of the top metal elements 12. This process can be described as governed by an effective capacitance C. As the charges travel back and forth, in response to the radio-frequency field, they flow around a long path through the vias 16 and the bottom ground plane 14. Associated with these currents is a magnetic field, and thus an inductance L. The effective circuit elements are illustrated in FIG. 2. The capacitance is controlled by the proximity of the adjacent metal elements 12, while the inductance is controlled by the thickness of the structure (i.e. the distance between the metal elements 12 and the ground plane 14).
The presence of an array or lattice of resonant LC circuits affects the reflection phase of the Hi-Z surface. For frequencies far from resonance, the surface reflects radio frequency waves with a xcfx80 phase shift, just as an ordinary conductor does. However, at the resonant frequency, the surface reflects with a zero phase shift. As a frequency of the incident wave is tuned through the resonant frequency of the surface, the reflection phase changes by one complete cycle, or 2xcfx80. This is seen in both the calculated and measured reflection phases, as shown in FIGS. 3 and 4, respectively. FIG. 3 shows the calculated reflection phase of the high-impedance surface, obtained from the effective medium model. The phase crosses through zero at the resonance frequency of the structure. FIG. 4 shows that the measured reflection phase agrees well with the calculated reflection phase reinforcing the validity of the effective medium model.
When the reflection phase is near zero, the structure also effectively suppresses surface waves, which has been shown to be significant in antenna applications.
Structures of this type have been constructed in a variety of forms, including multi-layer versions with overlapping capacitor plates. Examples have been demonstrated with resonant frequencies ranging from hundreds of megahertz to tens of gigahertz, and the effective media model presented herein has proven to be an effective tool for analyzing and designing these materials, now known as Hi-Z surfaces.
The present invention involves a method and apparatus for tuning the reflection phase of the Hi-Z surface using a material which locally changes its dielectric constant in response to external stimuli. Liquid crystal materials can be used as the material which locally changes its dielectric constant. Alternatively, instead of liquid crystal materials, one can use suspended microtubules, suspended metal particles, ferroelectrics, or any other media which has an electrically, for example, tunable dielectric constant. Since this device is electronically reconfigurable, it requires no macroscopic mechanical motion. Instead, it uses electric field-induced molecular reorientation within a layer of liquid crystal material or other appropriate material to produce an electrically tunable capacitance. Tunable capacitors make up resonant elements which are distributed across the Hi-Z surface, and determine the reflection phase at each point on the surface. By varying the reflection phase as a function of position, a reflected wave can be steered electronically. In addition, this method and apparatus can be combined with mechanical techniques to create a hybrid structure which can allow for even more tunability.
Important features of the present invention include:
1. A structure which incorporates a liquid crystal material or other tunable material into the capacitive region of a Hi-Z surface to produce a surface with tunable reflection phase.
2. The disclosed structure and methods can be used to extend the useful bandwidth of a Hi-Z surface.
3. A method of steering or focusing a microwave or radio-frequency beam using a structure having a Hi-Z surface and a media which has an electrically tunable dielectric constant, such as a liquid crystal.
The present invention can be applied to a wide range of microwave and millimeter-wave antennas were quasi-optical elements can improve performance. The present invention has application in space-based radar and airborne communications node (ACN) systems whereby an aperture must be continually reconfigured for various functions. The present invention can be used to replace a fixed reflector with an adaptive planar reflector, and provide for beam direction and tracking. They are also many commercial applications for multi-functional apertures of the type which can be produced using the invention as disclosed wherein.
In one aspect the present invention provides a tuneable impedance surface for steering and/or focusing an incident radio frequency beam, the tunable surface comprising: a ground plane; a plurality of elements disposed a distance from the ground plane, the distance being less than a wavelength of the radio frequency beam; and a capacitor arrangement for controllably varying the capacitance of adjacent elements, the arrangement including a dielectric material which locally changes its dielectric constant in response to an external stimulus.
In another aspect the present invention provides a method of tuning a high impedance surface for a radio frequency signal. The method includes arranging a plurality of generally spaced-apart planar conductive surfaces in an array disposed essentially parallel to and spaced from a conductive back plane, the size of each conductive surface being less than a wavelength of the radio frequency signal and the spacing of each conductive surface from the back plane being less than a wavelength of the radio frequency signal; and varying the capacitance between adjacent conductive surfaces by locally varying a dielectric constant of a dielectric material to thereby tune the impedance of said high impedance surface.